The present invention relates to a control circuit for a transmission output of a radio transmitter or the like, and more particularly, to a control circuit suitable for a TDMA transmitter requiring stable control with high precision over a wide dynamic range.
Heretofore, the control circuit for the transmission output of this type frequently uses a closed-loop system which detects part of the transmission output to feed the detected output back to a power amplifier (see for example, U.S. Pat. No. 4,602,218).
The control circuit for the transmission output as shown in FIGS. 11 and 12, for example, are known.
In FIG. 11, an output signal produced by an oscillation source 1 is supplied to a power amplifier 3 (power amplifier system) to drive the power amplifier so that a transmission output (TX) is obtained. Part of the transmission output is taken out by a directional coupler 4 and is detected by a detector 7. A detected output (Vdet) of the detector 7 is compared with a reference output (Vref) in a comparison error amplifier 9 and an output of the comparison error amplifier 9 controls the power amplifier 3 as a control voltage (Vcont). In this manner, a feedback system (detection system) is formed. Numeral 8 represents a waveform of a reference output in the form of burst.
Further, in Japanese Patent Application No. 2-219215 (JP-A-4-100426) shown in FIG. 12, a variable attenuator 5 is connected between the directional coupler 4 and the detector 7 to vary an amount of attenuation of the variable attenuator 5 in accordance with a value of the transmission output, so that an input level of the detector 7 is maintained constant so that the detection system can attain stable control over a wide dynamic range.
However, in the first conventional circuit shown in FIG. 11, when it is applied to a TDMA transmitter to produce the transmission output in the form of burst, it is difficult to make short rising and falling characteristics of the transmission output and adjust the transmission output over a wide range since dynamic ranges of both of the detection system and the power amplifier system are narrow. Further, there is a problem that the stability including a temperature characteristic of an output is lacking in a small output.
In the second conventional circuit configuration shown in FIG. 12, a dynamic range of control of the detection system is broadened and accordingly the stability and the reproducibility are improved, while since a dynamic range of the power amplifier system is the same as that of FIG. 11, there is a problem that overshoot and unnecessary oscillation occur in rising of the burst output. Further, the stability of a temperature characteristic of the transmission output is not sufficient.